OPTIMIZING HIGH-ORDER HARMONIC-GENERATION IN STRONG FIELDS

We present high-order harmonic generation results obtained with different laser systems, a 1 ps 1053 nm Nd-glass laser, a 2 ps 616 nm synchronously pumped dye laser, a 36 ps 1064 nm mode-locked Nd-YAG laser and the second harmonics (308 nm and 532 nm) of the latter two systems, We investigate the influence of the laser pulse width, the excitation wavelength (from the near infrared to the ultraviolet) and the atomic medium on the number of photons produced and on the maximum energy attained. Harmonic generation also depends strongly on the focusing conditions. By using simple arguments and results of numerical calculations in xenon, we show that the conversion efficiency in general follows a simple b3 power law, b denoting the laser confocal parameter, up to a transition regime where the coherence length of the process becomes equal to the medium length. By applying the b3 scaling as a normalization factor, we can then compare experimental results obtained in different focusing geometries. Our experimental data show that the optimization of the photon energy produced is favoured by using long incident wavelengths and light atomic systems with a high ionization energy. On the other hand, large numbers of photons are obtained with short incident wavelengths for low-order processes. We report the observation of the 53rd harmonic in neon with the 1053 nm Nd-glass laser at an intensity of 5 x 10(14) W cm-2 and the measurement of a conversion efficiency as high as 5 x 10(-7) for the 19th harmonic (22 eV) of the same laser in xenon. with a power of about 3 kW.